U.S. patent number 7,728,100 [Application Number 11/992,163] was granted by the patent office on 2010-06-01 for process for producing polyglycolic acid resin composition.
This patent grant is currently assigned to Kureha Corporation. Invention is credited to Fumio Akutsu, Fuminori Kobayashi, Katsushi Momose, Hiroyuki Sato.
United States Patent |
7,728,100 |
Sato , et al. |
June 1, 2010 |
Process for producing polyglycolic acid resin composition
Abstract
A particulate polyglycolic acid resin composition suitable as a
material for various forming processes in produced through a
process characterized by comprising: cooling a polyglycolic acid
resin composition having a residual glycolide content of at most
0.6 wt. % in a molten state by contact with an aqueous cooling
medium to solidify the composition, and pelletizing the
composition.
Inventors: |
Sato; Hiroyuki (Iwaki,
JP), Kobayashi; Fuminori (Iwaki, JP),
Akutsu; Fumio (Iwaki, JP), Momose; Katsushi
(Tokyo, JP) |
Assignee: |
Kureha Corporation (Chuo-ku,
Tokyo, JP)
|
Family
ID: |
37888848 |
Appl.
No.: |
11/992,163 |
Filed: |
September 20, 2006 |
PCT
Filed: |
September 20, 2006 |
PCT No.: |
PCT/JP2006/318581 |
371(c)(1),(2),(4) Date: |
March 18, 2008 |
PCT
Pub. No.: |
WO2007/034805 |
PCT
Pub. Date: |
March 29, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090118462 A1 |
May 7, 2009 |
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Foreign Application Priority Data
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Sep 21, 2005 [JP] |
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2005-273958 |
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Current U.S.
Class: |
528/272; 264/5;
562/587; 560/185; 560/179; 528/361; 528/271 |
Current CPC
Class: |
C08G
63/90 (20130101); C08G 63/89 (20130101); C08G
63/06 (20130101); C08K 5/005 (20130101); C08K
5/29 (20130101) |
Current International
Class: |
C08G
63/02 (20060101); C08G 63/00 (20060101) |
Field of
Search: |
;264/5 ;528/271,272,361
;560/179,185 ;562/587 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2000-302855 |
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Apr 1999 |
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JP |
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2000-302855 |
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Oct 2000 |
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JP |
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2001-064400 |
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Mar 2001 |
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JP |
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2005-053870 |
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Aug 2003 |
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JP |
|
2005-246718 |
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Mar 2004 |
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JP |
|
2005-162873 |
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Jun 2005 |
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JP |
|
2005-246718 |
|
Sep 2005 |
|
JP |
|
WO2005/090438 |
|
Sep 2005 |
|
WO |
|
Other References
International Search Report of PCT/JP2006/318581 mailed Oct. 17,
2006. cited by other .
PCT International Preliminary Report on Patentability and Written
Opinion of the International Searching Authority mailed Sep. 9,
2008, in English. cited by other.
|
Primary Examiner: Boykin; Terressa M
Attorney, Agent or Firm: Reed Smith LLP
Claims
The invention claimed is:
1. A process for producing a particulate polyglycolic acid resin
composition, comprising: melt-extruding 100 wt. parts of a
polyglycolic acid resin together with at most 3 wt. parts of a
thermal stabilizer selected from the group consisting of alkyl
phosphate and phosphite esters each having an alkyl group of 8-24
carbon atoms to form a polyglycolic acid resin composition having a
residual glycolide content of at most 0.6 wt. % in a molten state,
cooling the polyglycolic acid resin composition by contact with an
aqueous cooling medium to solidify the composition, and pelletizing
the composition.
2. A production process according to claim 1, further including a
step of crystallizing the PGA resin after the solidification and
before and/or after the pelletization.
3. A production process according to claim 1 wherein the aqueous
cooling medium is in a temperature range of from ca. 5.degree. C.
to ca. 100.degree. C.
4. A production process according to claim 1 wherein the aqueous
cooling medium is water.
5. A production process according to claim 1 wherein the
polyglycolic acid resin composition in a molten state has a
glycolide content of at most 0.3 wt. %.
6. A production process according to claim 1 wherein the
polyglycolic acid resin composition in a molten state contains a
carboxyl group-capping agent.
7. A particulate polyglycolic acid resin composition produced
through a production process according to claim 1 and having a
variation coefficient of particle size of at most 7%.
8. A particulate polyglycolic acid resin composition according to
claim 7, exhibiting a molecular weight-retention percentage of at
least 20% after 3 days of storage in an environment of 50.degree.
C. and 90%.
Description
This application is the United States national stage of
International Application No. PCT/JP2006/318581, filed Sep. 20,
2006, which was published under PCT Article 21 in Japanese as
International Publication No. WO/2007/034805 and which claims
benefit of Japanese Application No. 2005-273958 filed Sep. 21,
2005.
TECHNICAL FIELD
The present invention relates to a process for producing a
particulate polyglycolic acid resin composition which is suitable
as a starting material for various forming processes, such as
injection molding, film forming, sheet forming, and blow
molding.
BACKGROUND ART
Among aliphatic polyesters considered to give little load to the
natural environment due to their biodegradability or
hydrolyzability, polyglycolic acid resin has a particularly
extensive decomposability, is excellent in mechanical strengths
such as tensile strength and in gas-barrier property when formed
into a film or sheet, and is therefore expected to be used as
fishery materials, such as fishing yarns, fishery nets and
culturing nets, agricultural material, or various packaging
(container) materials (e.g., Patent documents 1-3 listed below).
However, the severe hydrolyzability of polyglycolic acid resin has
frequently provided a factor of obstructing the application
thereof. For example, in the case of obtaining various forms, such
as filament, film and sheet, an ordinary thermoplastic resin is
generally melted, cooled, solidified and pelletized to obtain a
particulate forming resin material, which is then supplied for
various forming processes. However, regarding the cooling step
among the above-mentioned series of steps, cooling with water has
been believed impossible or impractical for polyglycolic acid resin
having severe hydrolyzability. This is because polyglycolic acid
resin is hydrolyzed in a stage when a melt thereof is introduced
into water to be cooled, thus resulting in a polyglycolic acid
resin having a lower molecular weight which slows a non-ignorable
degree of lowering in moisture resistance (i.e., a further lowering
in molecular weight due to further hydrolysis under the condition
of use thereof and an accompanying lowering in practical strength).
Incidentally, Patent document 4 listed below discloses a method of
crystallization of aliphatic polyesters including a polyglycolic
acid resin, comprising: causing an aliphatic polyester in a solid
state, a molten state or a solution state to contact a liquid
including water to solidify and crystallize the aliphatic
polyester, but no working example is disclosed regarding
polyglycolic acid resin. For the above-mentioned reason, the
cooling subsequent to melting for formation of particulate forming
material of polyglycolic acid resin has been effected by air
cooling (i.e., cooling by contact with air). According to the
present inventors' experience, however, when a melt-extrudate
(strand) of polyglycolic acid resin is solidified by air cooling,
for example, the strands after solidification are distorted and,
when pelletized (e.g., into cylindrical pellets) by means of a
cutter, etc., provide a particulate product having a broad particle
size distribution, which is inconvenient for a subsequent product
forming step. Further, when the number of strands is increased in
order to increase the productivity, inconveniences, such as
entanglement and adhesion of the strands, are liable to occur.
Patent document 1: WO2003/037956A1 Patent document 2: JP10-60136A
Patent document 3: WO2005/072944A1 Patent document 4:
JP2000-302855A
DISCLOSURE OF INVENTION
Accordingly, a principal object of the present invention is to
provide a particulate polyglycolic acid resin composition suitable
as a starting material for various forming processes, particularly
a process for effective production thereof.
According to the study of the present inventors, it has been
discovered that the above-mentioned hydrolysis of molten
polyglycolic acid resin in the water cooling step is associated
with the glycolide content in the molten polyglycolic acid resin
and, if the glycolide content is suppressed to a certain level or
below by an appropriate means, the hydrolysis and lowering in
moisture resistance of the polyglycolic acid resin during water
cooling can be suppressed within a tolerable extent, and it becomes
also possible to uniformize the particle size distribution of the
particulate product.
The process for producing a particulate polyglycolic acid resin
composition is based on the above findings and comprises: cooling a
polyglycolic acid resin composition having a residual glycolide
content of at most 0.6 wt. % in a molten state by contact with an
aqueous cooling medium to solidify the composition, and pelletizing
the composition.
According to the process of the present invention, it becomes
possible to effectively produce a polyglycolic acid resin
composition having a narrow particle size distribution as
represented by a variation coefficient of particle size of at most
7%.
BEST MODE FOR PRACTICING THE INVENTION
Hereinbelow, the process for producing a particulate polyglycolic
acid resin composition according to the present invention will be
described with reference to preferred embodiments thereof.
(Polyglycolic Acid Resin)
The polyglycolic acid resin (hereinafter, sometimes referred to as
"PGA resin") includes homopolymer of glycolic acid (PGA, inclusive
of a ring-opening polymerization product of glycolide (GL) which is
a bimolecular cyclic ester of glycolic acid) consisting only of
glycolic acid recurring unit represented by a formula:
--(O.CH.sub.2.CO)-- . . . (1), and also a glycolic acid copolymer
containing at least 70 wt. % of the above-mentioned glycolic acid
recurring unit.
Examples of comonomers for providing the polyglycolic acid
copolymer together with the glycolic acid monomer such as
glycolide, may include: cyclic monomers, inclusive of ethylene
oxalate (i.e., 1,4-dioxane-2,3-dione); lactides; lactones, such as
.beta.-propiolactone, .beta.-butyrolactone; pivalolactone,
.gamma.-butyrolactone, .delta.-valerolactone,
.beta.-methyl-.delta.-valerolactone, and .epsilon.-caprolactone;
carbonates, such as trimethylene carbonate; ethers, such as
1,3-dioxane; ether-esters, such as dioxanone; and amides, such as
.epsilon.-caprolactam; hydroxycarboxylic acids, such as lactic
acid, 3-hydroxypropanoic acid, 4-hydroxybutanonic acid and
.delta.-hydroxycaproic acid, and their alkyl esters; substantially
equal molar mixtures of aliphatic diols, such as ethylene glycol
and 1,4-butane diol with aliphatic dicarboxylic acids, such as
succinic acid and adipic acid, and their alkyl or aromatic esters;
and two or more species of these. These monomers may be replaced by
polymers thereof which can be used as a starting material for
providing a polyglycolic acid copolymer together with the
above-mentioned glycolic acid monomer such as glycolide.
The above-mentioned glycolic acid recurring unit should occupy at
least 70 wt. %, preferably at least 90 wt. %, of the PGA resin. If
the content is too small, the strength or the gas-barrier property
expected of PGA resin becomes scarce. As far as this is satisfied,
the PGA resin can comprise two or more species of polyglycolic acid
(co)polymers in combination.
The PGA resin may preferably have a molecular weight (Mw
(weight-average molecular weight based on polymethyl methacrylate)
of 3.times.10.sup.4-8.times.10.sup.5, particularly
5.times.10.sup.4-5.times.10.sup.5, as measured by GPC measurement
using hexafluoroisopropanol solvent. If the molecular weight is too
small, the resultant form product is liable to have an insufficient
strength. On the other hand, too large a molecular weight is liable
to result in difficulties in melt-extrusion, forming and
processing.
The polyglycolic acid resin composition (hereinafter, also referred
to as "PGA resin composition") of the present invention can
comprise the above-mentioned PGA resin alone but may preferably
contain a carboxyl group-capping agent and/or a thermal stabilizer
in order to improve the moisture resistance and thermal stability
of the final form product. These additives can be mixed with the
particulate PGA resin obtained by the present invention to provide
a forming material prior to the forming, but may preferably be
added prior to the pelletization of the PGA resin composition of
the present invention. Particularly, by adding a thermal stabilizer
prior to the melting of the PGA resin, it becomes possible to
attain an effect of suppressing the increase of glycolide content
during the melting (and mixing) process of the PGA resin
composition before the water cooling step in the process of the
present invention.
As the carboxyl group-capping agent, it is generally possible to
use compounds having a function of capping a carboxyl terminal and
known as an agent for improving moisture resistance of aliphatic
polyesters, such as polylactic acid. Examples thereof may include:
carbodiimide compounds inclusive of monocarbodiimides and
polycarbodiimides, such as N,N-2,6-diisopropylphenylcarbodiimide;
oxazoline compounds, such as 2,2'-m-phenylene-bis(2-oxazoline),
2,2'-p-phenylene-bis(2-oxazoline), 2-phenyl-2-oxagoline, and
styrene-isopropenyl-2-oxazoline; oxazine compounds, such as
2-methoxy-5,6-dihydro-4H-1,3-oxazine; and epoxy compounds, such as
N-glycidylphthalimide, cyclohexene oxide, and tris
(2,3-epoxypropyl) isocyanurate. Among these, carbodiimide compounds
and epoxy compounds are preferred. These carboxyl group-capping
agents can be used in combination of two or more species as
desired, and may preferably be used in a proportion of 0.01-10 wt.
parts, further preferably 0.1-2 wt. parts, particularly preferably
0.2-1 wt. part, per 100 wt. parts of the PGA resin.
Further, preferred examples of the thermal stabilizer may include:
phosphoric acid esters having a pentaerythritol skeleton and alkyl
phosphate or phosphite esters having an alkyl group of preferably
8-24 carbon atoms, and some preferred specific examples thereof are
disclosed in WO2003/037956A1 (the disclosure of which is intended
to be incorporated herein by reference). These thermal stabilizers
may preferably be used in a proportion of at most 3 wt. parts, more
preferably 0.003-1 wt. part, per 100 wt. parts of the PGA
resin.
According to the process of the present invention, the
above-mentioned PGA resin composition is subjected to melting (and
mixing) by heating to a temperature range of preferably
230-280.degree. C., more preferably 240-270.degree. C. The melting
(and mixing) means may basically be any one, inclusive of a
stirring machine and a continuous kneader, but may preferably
comprise an extruder (e.g., an equi-directionally rotating
twin-screw extruder) allowing a short-time processing and a smooth
transfer to a subsequent cooling step for the heat-melting (and
mixing) therein. If the heat-melting temperature is below
230.degree. C., the effect of additives, such as the carboxyl
group-capping agent and thermal stabilizer, is liable to be
insufficient. On the other hand, in excess of 280.degree. C., the
PGA resin composition is liable to be colored.
According to the present invention, the glycolide content in the
molten PGA resin composition after the melting (and mixing) and
prior to the cooling with an aqueous cooling medium, is suppressed
to at most 0.6 wt. %, preferably at most 0.3 wt. %, whereby the
hydrolysis of the PGA resin during the water cooling is suppressed.
For the suppression of the glycolide content in the molten PGA
resin composition, any of (a) lowering in glycolide content in the
starting PGA resin, (b) the incorporation of a thermal stabilizer
as descried above, and (c) discharge of glycolide having a
relatively low boiling point, e.g., through a vent port of an
extruder for the melt-mixing, is effective, and by appropriately
combining these measures, the glycolide content of at most 0.6 wt.
% is accomplished. It is particularly preferred to lower the
glycolide content of the starting PGA resin composition (a) in
advance to below 0.5 wt. %, further at most 0.3 wt. %, particularly
at most 0.2 wt. %. In order to obtain such a PGA resin having a low
glycolide content, it is preferred to apply a ring-opening
polymerization of glycolide wherein at least a latter period of the
polymerization is proceeded by way of solid-phase polymerization,
and the resultant PGA resin is subjected to removal of glycolide by
release to a gas phase (as disclosed in WO2005/090438A1).
As the aqueous cooling medium for contact cooling and
solidification of the molten PGA resin composition having a
suppressed glycolide content in the above-mentioned manner, it is
possible to use water alone, or a mixture of water with a solvent
mutually soluble with water, such as an alcohol or an ester. From
the viewpoints of environmental sanitation, economy, heat
efficiency, etc., water alone (tap water or deionized water) is
preferred, but it is possible to add a soap, a surfactant, etc., in
a small quantity not adversely affecting the object of the present
invention.
The molten PGA resin composition is cooled and solidified by
contact with a aqueous cooling medium sprayed thereto or by
introduction or immersion in a bath of aqueous cooling medium.
The temperature of the aqueous cooling medium may generally be in
the range of ca. 5.degree. C. to ca. 100.degree. C. In view of the
economy and cooling efficiency, room temperature can be used
without any problem, but the temperature increase of the medium by
contact with the molten PGA resin composition need not be
suppressed by forced cooling. Particularly, in order to smoothly
proceeding with a subsequent pelletization step, it is also
preferred to promote the crystallization of the PGA resin
composition at this stage, and for this purpose, it is preferred
that the temperature of the aqueous cooling medium is within a
temperature range of the glass transition temperature (Tg) of the
PGA resin composition .+-.30.degree. C. It is also preferred, for
example, to introduce the strands of molten PGA resin into an
aqueous cooling medium at a temperature around such a glass
transition temperature (ordinarily 40-50.degree. C.) of the PGA
resin to effect the cooling for a relatively short period, and
withdraw the PGA resin having an inner portion thereof in a stage
of yet insufficient cooling to proceed with the crystallization of
the PGA resin by heat from the inner portion thereof.
The molten PGA resin composition may be cooled by introduction into
a bath of aqueous cooling medium or by spraying of aqueous cooling
medium. From the viewpoint of cooling efficiency, a countercurrent
contact between the bath of aqueous cooling medium and the molten
extruded strands of PGA resin composition is preferred but, in this
case, the temperature of the PGA resin composition finally
contacting the cooling bath becomes close to the temperature of the
introduced aqueous cooling medium, and this is not desirable from
the viewpoint of promotion of the crystallization. Accordingly, it
is also preferred to effect a concurrent flow contact with a bath
of aqueous cooling medium or spraying of a circulating aqueous
cooling medium at an elevated temperature.
In any case, if the crystallization in the cooling step of PGA
resin composition is insufficient, it is possible to place an
additional step for promoting the crystallization after
solidification by the cooling and before the pelletization and/or
after the pelletization. The medium for this purpose may suitably
be an aqueous medium at a temperature in a temperature range of
from the Tg of PGA resin composition to ca. 100.degree. C., or
heated air at a somewhat higher temperature.
The cooling and pelletization of the PGA resin composition can be
effected simultaneously, if the molten PGA resin composition is
pelletized by spraying or dispersion with a rotating disk and then
caused to contact an aqueous cooling medium. However, it is further
preferred, as described above, to cool and solidify the strands of
molten PGA resin composition by contact with an aqueous cooling
medium, followed by optional crystallization, and pelletization.
The pelletization (into cylindrical pellets) of the cooled and
solidified strands may be performed, e.g., by means of a
cutter.
The particulate PGA resin composition, preferably pelletized in the
form of pellets, thus obtained through the process of the present
invention, may have a particle size (in the case of a pellet, a
diameter of a true sphere having an identical volume) in a range of
1-4 .mu.m and a narrow particle size distribution as represented by
a variation coefficient of particle size (=standard
deviation/number-average particle size) of at most 7%, and is
suitably used as a starting material for various forming processes,
such as injection molding and blow molding. Further, it may have a
moisture resistance as represented by a molecular weight retention
percentage of at least 20%, particularly at least 30%, after
storage for 3 days in an environment of 50.degree. C., 90%.
In the present invention, it is possible to use a filler in order
to impart a mechanical strength and other properties to the PGA
resin composition. The filler is not particularly limited in
species but may be in the form of fiber, plates, powder or
particles. Specific examples thereof may include: fiber or whisker
form fillers, such as glass fiber, PAN-based and pitch-based carbon
fiber metal fiber, such as stainless steel fiber, aluminum fiber
and brass fiber, natural fiber of chitin, chitosan, cellulose,
cotton, etc., organic synthetic fiber such as aromatic polyamide
fiber, gypsum fiber, ceramic fiber, asbestos fiber, zirconia fiber,
alumina fiber, silica fiber, titanium oxide fiber, silicon carbide
fiber, rock wool, potassium titanate whisker, barium titanate
whisker, aluminum borate whisker, and silicon nitride whisker; and
powdery, particulate and plate-like fillers of natural inorganic
minerals, such as mica, talc, kaolin, silica and sand, calcium
carbonate, glass beads, glass flake, glass micro-balloon, clay,
molybdenum disulfide, wallastenite, montmorillonite, titanium
oxide, zinc oxide, calcium polyphosphate and graphite. Any type of
glass fiber can be used without particular restriction as far as it
is generally usable for reinforcement of resins, and can be
selected from chopped strands of long fiber type and short fiber
type, and milled fiber. The above-mentioned fillers can be used in
two or more species in combination. Incidentally, these fillers can
be used after surface treatment thereof with known coupling agents,
such as silane coupling agents and titanate coupling agents, and
other surface treating agents. Further, the glass fiber can be
coated or bundled with a thermoplastic resin, such as
ethylene/vinyl acetate copolymer, or a thermosetting resin such as
epoxy resin. The filler may be added in 0.1-100 wt. parts,
preferably 1-50 wt. parts, per 100 wt. parts of the PGA resin.
EXAMPLES
Hereinbelow, the present invention will be described more
specifically based on Examples and Comparative Examples. The
physical properties (or values) described in the present
specification including the following description are based on
those measured according to the following methods.
(1) Glycolide Content
To ca. 100 mg of a sample PGA resin(composition), 2 g of dimethyl
sulfoxide containing 4-chlorobenzophenone as an internal standard
at a concentration of 0.2 g/l, was added, and the mixture was
heated at 150.degree. C. for ca. 5 min. to dissolve the resin and,
after being cooled to room temperature, was subjected to
filtration. Then, 1 .mu.l of the filtrate solution was taken and
injected into a gas chromatography (GC) apparatus for measurement.
From a value obtained from the measurement, a glycolide content was
calculated in terms of wt. % contained in the polymer. The GC
analysis conditions were as follows.
Apparatus: "GC-2010" made by K. K. Shimadzu Seisakusho) Column:
"TC-17" (0.25 mm in diameter.times.30 mm in length). Column
temperature: Held at 150.degree. C. for 5 min., heated at
270.degree. C. at a rate of 20.degree. C./min. and then held at
270.degree. C. for 3 min. Gasification chamber temperature:
180.degree. C. Detector: FID (hydrogen flame ionization detector)
at temperature of 300.degree. C.
In the present invention, the glycolide content in a PGA resin
after heat-melting and before cooling is controlled. However, no
substantial change in glycolide content was confirmed before or
after the cooling, a glycolide content in pellets after cooling was
used to represent a glycolide content in PGA resin before
cooling.
(2) Molecular Weight Measurement
Ca. 5 g of a sample PGA resin (composition) was sandwiched between
aluminum plates and heated for 3 minutes on a heat press machine at
260.degree. C. Then, the sample was held for ca. 5 minutes under a
pressure of 5 MPa, then immediately transferred to a press machine
cooled with circulating water and held for ca. 5 minutes under a
pressure of 5 MPa to form a transparent amorphous sheet.
From the above-prepared press sheet, ca. 10 mg of a sample was cut
out and was dissolved in 10 ml of hexafluoroisopropanol (HFIP)
containing sodium trifluoroacetate dissolved therein at 5 mM. Then,
20 .mu.l of the resultant sample solution was filtrated through a
0.1 .mu.m-membrane filter made of polytetrafluoroethylene and then
injected into a gel permeation chromatography (GPC) apparatus for
measurement of molecular weight under the following conditions.
Incidentally, the sample was injected into the GPC apparatus within
30 minute after the dissolution.
<GPC Measurement Conditions> Apparatus: "Shodex-104" made by
Showa Denko K.K. Column: Two columns of "HFIP-606M" were connected
in series with 1 column of "HFIP-G" as a pre-column. Column
temperature: 40.degree. C. Elution liquid: HFIP solution containing
sodium trifluoroacetate dissolved at 5 mM. Flow rate: 0.6 ml/min.
Detector: RI (differential refractive index) detector. Molecular
weight calibration: Effected by using 5 species of standard
polymethyl methacrylate having different molecular weights.
(3) Moisture Resistance Evaluation
Ca. 1 g of a pellet sample was sandwiched between aluminum plates
and heated for 3 minutes on heat press machine at 260.degree. C.
Then, the sample was held for 1 minute under a pressure of 5 MPa
and then immediately transferred to a press machine cooled with
circulating water to be cooled to form a transparent amorphous
press sheet. The press sheet thus formed was then heat-treated for
10 minutes at 80.degree. C. in the state of being sandwiched
between the aluminum plates.
Ca. 10 mg of a sample was cut out from the press sheet prepared
through the above operation and held for 3 days in a constant
temperature and humidity chamber held at a temperature of
50.degree. C. and relative humidity of 90%. The sample was taken
out after the 3 days and measured with respect to a molecular
weight by gel permeation chromatography (GPC). A molecular weight
retentivity was calculated from the measured molecular weight and a
molecular weight of sample before being placed in the constant
temperature and humidity chamber, and a moisture resistance was
evaluated based on the molecular weight retentivity.
[Carboxyl Group Concentration]
From a press sheet prepared in the same manner as a sample for
evaluating moisture resistance, a sample was cut, accurately
weighed at ca. 0.3 g and completely dissolved in 10 ml of dimethyl
sulfoxide of a reagent grade on an oil bath for ca. 3 min. Two
drops of an indicator (0.1 wt. % Bromothymol Blue/methyl alcohol
solution) was added, and further a 0.02 normal-sodium
hydroxide/benzyl alcohol solution was gradually added thereto until
a termination point where the color of the solution changed from
yellow to green by observation with eyes. From the amount of the
dropped sodium hydroxide solution, a carboxyl group concentration
was calculated in terms of equivalents per t (ton) of PGA
resin.
(4) Variation Coefficient of Particle Size
From a produced PGA resin pellet product, 20 cylindrical pellets
were selected at random, their particle sizes were determined for
each cylindrical pellet as a diameter of a true sphere having an
identical volume, and from the determined particle sizes, a
variation coefficient of particle size was calculated as a standard
deviation of particle size/a number-average particle size.
(5) State of Strands
A state of melt-extruded strands of PGA resin composition and a
state of the strands after cooling and before pelletization were
observed with eyes and evaluated according to the following
standard:
A: The strands were free from distortion and stable.
C: The strands were distorted and unstable.
(Synthesis Example of PGA Resin)
Into a closable vessel (glycolide dissolution vessel) equipped with
a steam jacket structure, 22.5 kg of glycolide, 0.68 g (30 ppm) of
tin chloride dihydrate and 21.8 g of n-dodecyl alcohol were added
and, after closing the vessel, steam was circulated in the jacket
to melt the contents by heating up to 100.degree. C., thereby
forming a uniform liquid. While keeping the temperature of the
contents at 100.degree. C., they were transferred to an apparatus
comprising tubes made of a metal (SUS304) and each having an inner
diameter of 24 mm. The apparatus was composed of a body part
including the tubes, and upper and lower plates. The body part and
the upper and lower plates were respectively provided with jackets,
through which a heat transfer oil could be circulated. When the
contents were transferred to this apparatus which had been provided
with the lower plate and, immediately after the transfer, the upper
plate was affixed.
A heat transfer oil at 170.degree. C. was circulated into the
jackets of the body part and the upper and lower plates, and the
contents were held for 7 hours to obtain a PGA resin having a
weight-average molecular weight of 2.3.times.10.sup.5 and a
glycolide content of 0.2 wt. %.
Example 1
To the PGA resin obtained in the above Synthesis Example, 300 ppm
of an almost equi-molar mixture of mono- and di-stearyl acid
phosphate ("AX-71" (trade name), made by Asahi Denka Kogyo K. K.)
was added, and the resultant mixture was melt-kneaded and extruded
through a twin-screw kneading extruder ("LT-20", made by K. K. Toyo
Seiki Seisakusho) having zones C1-C4 from the feed port to the
discharge port set at temperatures of 220.degree. C., 230.degree.
C., 240.degree. C. and 230.degree. C., respectively, to form a
strand of the PGA resin through a die having a single-strand bore,
which strand was then introduced into a water bath filled with
water at ca. 65.degree. C. to be cooled for 5 seconds to be
solidified. Then, the solidified strand was taken up from the water
bath, exposed to air at ca. 25.degree. C. for ca. 10 seconds and
pelletized (into cylindrical pellets) by a pelletizer equipped with
a rotary cutter while being pulled up at a constant speed, to
obtain PGA pellets having an average particle size of 1.5 mm, a
standard deviation of 0.05 mm and a variation coefficient of 3%, a
glycolide content of 0.19 wt. %, a weight-average molecular weight
of 2.31.times.10.sup.5 and a carboxyl group concentration of 9
equivalents/t.
As an intermediate state, it was confirmed by eye observation that
the melt-extruded strand was not distorted but straight during the
cooling and stably conveyed to the pelletizer to be pelletized
(into cylindrical pellets). The strand after the cooling tuned into
white and the progress of the crystallization thereof was
confirmed. As a result of the moisture resistance evaluation, the
thus-obtained pellets exhibited a molecular weight of
1.13.times.10.sup.3 and thus a molecular weight-retention of 49%
after 3 days of storage at 50.degree. C. and 90%-relative
humidity.
The cooling condition and property evaluation of the above Example
and summarized in Table 1 appearing hereinafter together with
Examples and Comparative Examples described below.
Comparative Example 1
PGA resin pellets were prepared and evaluated in the same manner as
in Example 1 except that the PGA resin strand extruded out of the
die of the extruder was placed on a meshed conveyer and blown twice
with dry air (dew point=-50.degree. C.) at ca. 25.degree. C. for
ca. 15 seconds in air to be solidified. As an intermediate state,
it was confirmed by eye observation that the melt-extruded strand
was distorted during the cooling on the conveyer and jumped up from
the conveyer, thus being unstable. The strand after the cooling
turned white similarly as in Example 1 and the progress of the
crystallization thereof was confirmed.
Example 2
PGA resin pellets were prepared and evaluated in the same manner as
in Example 1 except that the thermal stabilizer "AX-71" was
replaced by 300 ppm of cyclic neopenta-tetra-il-bis(octadecyl
phosphite) ("ADEKASTAB PEP-8" (trade name), made by Asahi Denka
Kogyo K. K.). The strand during the cooling was stable similarly as
in Example 1 and the whitening thereof was confirmed by observation
with eyes.
Comparative Example 2
PGA resin pellets were prepared and evaluated in the same manner as
in Example 2 except that the PGA resin strand extruded out of the
die of the extruder was placed on a meshed conveyer and cooled with
dry air (dew point=-50.degree. C.) at ca. 25.degree. C. for ca. 15
seconds to be solidified similarly as in Comparative Example 1. The
strand during the cooling was unstable similarly as in Comparative
Example 1 and the whitening thereof was confirmed by observation
with eyes.
Example 3
PGA resin pellets were prepared and evaluated in the same manner as
in Example 1 except that, to the PGA resin, 0.5 wt. % of
N,N-2,6-diisopropylphenylcarbodiimide (made by Kawaguchi Kagaku
Kogyo K. K.) was added as a carboxyl group-capping agent to the PGA
resin in addition to the 300 ppm of the thermal stabilizer "AX-71".
The strand during the cooling was stable similarly as in Example 1
and the whitening thereof was confirmed by observation with
eyes.
Example 4
PGA resin pellets were prepared and evaluated in the same manner as
in Example 1 except that the strand extruded out of the die of the
extruder was placed on a mesh conveyer and introduced into a water
bath filled with water at ca. 15.degree. C. for 15 seconds of
cooling to be solidified, and the solidified strand was exposed to
air at ca. 25.degree. C. for ca. 10 seconds. It was confirmed by
observation with eyes that the strand during the cooling was stable
and conveyed to a pelletizer to be pelletized (into cylindrical
pellets) similarly as in Example 1. The strand after the cooling
was transparent and judged to be in an amorphous state.
Comparative Example 3
PGA resin pellets were prepared and evaluated in the same manner as
in Example 1 except that the thermal stabilizer "AX-71" was not
used. The strand during the cooling was stable similarly as in
Example 1 and the whitening thereof was confirmed by observation
with eyes.
Comparative Example 4
PGA resin were prepared and evaluated in the same manner as in
Comparative Example 3 except that the PGA resin strand extruded out
of the die of the extruder was cooled for 15 seconds with dry air
(dew point=-50.degree. C.) at ca. 25.degree. C. to be solidified.
The strand during the cooling was unstable similarly as in
Comparative Example 1 and the whiting thereof was confirmed by
observation with eyes.
Example 5
A starting composition prepared by adding, to the PGA resin, 0.5
wt. % of N,N-2,6-diisopropylphenylcarbodiimide (Kawaguchi Kagaku
Kogyo K. K.) as a carboxyl group-capping agent in addition to the
300 ppm of the thermal stabilizer "AX-71" similarly as in Example
3, was melt-kneaded by means of a twin-screw extruder ("TEX44
.alpha. II" made by Nippon Seikosho K. K.) having zones C1-C12 from
the supply part to the discharge part set to 50.degree. C. (C1),
180.degree. C. (C2), 260.degree. C. (C3-C9) and 230.degree. C.
(C10-C12), while effecting gas-evacuation at a vacuum of 0.2 torr
through a vent port provided between zones C10 and C11, and
extruded through a die equipped with 29 strand bores each in a
diameter of 4 mm as PGA resin strands, which were introduced into a
water bath at ca. 65.degree. C. for 5 seconds of cooling to be
solidified. Then, the strands taken up onto a mesh-conveyer and
blown three times with air at ca. 25.degree. C. for ca. 10 seconds
of exposure to the air, and then pelletized (into cylindrical
pellets) by means of a cutter equipped with a rotary cutter while
being pulled at a constant speed, to obtain PGA resin pellets which
were then evaluated. It was confirmed by observation with eyes that
the strands during the cooling were substantially free from
distortion and straight and stably conveyed to the pelletizer to be
pelletized (into cylindrical pellets). The strands after the
cooling turned white similarly as in Example 1, and the
crystallization thereof was confirmed.
Comparative Example 5
PGA resin pellets were prepared and evaluated in the same manner as
in Example 5 except that the PGA resin strands extruded out of the
die of the extruder were placed on a meshed conveyer and blown 5
times with dry air (dew point=ca.-50.degree. C.) for 30 seconds of
cooling to be solidified. It was confirmed by observation with eyes
that strands during the cooling were distorted to be unsettled on
the conveyer and often caused bonding between strands. The strands
after the cooling turned white similarly as in Example 1, and the
progress of the crystallization thereof was confirmed.
The outlines of the results of the above Examples and Comparative
Examples are inclusively shown in the following Table 1.
TABLE-US-00001 TABLE 1 Example 1 Comp. 1 2 Comp. 2 3 4 Comp. 3
Comp. 4 5 Comp. 5 Main cooling Cooling medium water air water air
water water water air water air Temperature (.degree. C.) 65 25 65
25 65 15 65 25 65 25 Time (sec.) 5 15 5 15 5 10 6 15 5 30 Glycolide
content (wt. %) 0.19 0.22 0.55 0.54 0.27 0.28 1.05 1.04 0.12 0.13
Evaluation Carboxyl group-concentration (eg./t) 9 9 11 9 3 3 13 9 3
3 State of strand A C A C A A A C A C Variation coefficient of
particle size (%) 3 10 5 9 6 4 4 10 3 10 Moisture resistance *: (%)
49 47 30 32 52 50 12 16 61 60 *: Molecular weight retention (%)
after storage for 3 days at 50.degree. C./99% RH.
INDUSTRIAL APPLICABILITY
As is understood from the results of Table 1, according to Examples
of the present invention, it was possible to efficiently produce
particulate PGA resin compositions having a narrow particle size
distribution and suitable as a material for various forming
processes, by subjecting a molten polyglycolic acid resin
composition having a glycolide content which has been reduced by
various means, such as lowering of glycolide content in the
starting PGA resin, addition of a thermal stabilizer and vent as an
auxiliary means, to water-cooling and solidification and
pelletization. In contrast thereto, Comparative Examples 1, 2, 4
and 5 having adopted air cooling instead of water cooling, all
caused instability of melt-extruded strands and much distortion
after cooling, which resulted in pellets having a broad particle
size distribution (a large variation coefficient of particle size).
Further, the PGA resin pellets obtained through water cooling of a
molten PGA resin composition having a large glycolide content
(Comparative Example 3) resulted in an increase in carboxyl group
concentration and a lower moisture resistance.
* * * * *